If you try to put an oversized thing in it, then it'll fall out right
away.

Options

max The maximum size of the cache, checked by applying the length
function to all values in the cache. Not setting this is kind of
silly, since that's the whole purpose of this lib, but it defaults
to Infinity.

maxAge Maximum age in ms. Items are not pro-actively pruned out
as they age, but if you try to get an item that is too old, it'll
drop it and return undefined instead of giving it to you.

length Function that is used to calculate the length of stored
items. If you're storing strings or buffers, then you probably want
to do something like function(n){return n.length}. The default is
function(n){return 1}, which is fine if you want to store n
like-sized things.

dispose Function that is called on items when they are dropped
from the cache. This can be handy if you want to close file
descriptors or do other cleanup tasks when items are no longer
accessible. Called with key, value. It's called before
actually removing the item from the internal cache, so if you want
to immediately put it back in, you'll have to do that in a
nextTick or setTimeout callback or it won't do anything.

stale By default, if you set a maxAge, it'll only actually pull
stale items out of the cache when you get(key). (That is, it's
not pre-emptively doing a setTimeout or anything.) If you set
stale:true, it'll return the stale value before deleting it. If
you don't set this, then it'll return undefined when you try to
get a stale entry, as if it had already been deleted.

API

set(key, value)

get(key) => value

Both of these will update the "recently used"-ness of the key.
They do what you think.

peek(key)

Returns the key value (or undefined if not found) without
updating the "recently used"-ness of the key.

(If you find yourself using this a lot, you might be using the
wrong sort of data structure, but there are some use cases where
it's handy.)

del(key)

Deletes a key out of the cache.

reset()

Clear the cache entirely, throwing away all values.

has(key)

Check if a key is in the cache, without updating the recent-ness
or deleting it for being stale.

forEach(function(value,key,cache), [thisp])

Just like Array.prototype.forEach. Iterates over all the keys
in the cache, in order of recent-ness. (Ie, more recently used
items are iterated over first.)